1. Field of the Invention
The present invention relates to an electronic image forming apparatus for forming an image or a character on a paper.
2. Description of the Related Art
In the conventional electronic photographic apparatus, for example, as shown in FIG. 13, a laser beam 33 applied from an exposing unit (not shown) serves to expose the surface of a photosensitive body 31 charged with a predetermined electric potential by a charger 32, for forming an electrostatic latent image according to an electric potential difference between an exposed portion where an electric potential is made lower and an unexposed portion where an electric potential is not made lower.
This electrostatic latent image is developed to be a toner image by a developing unit 34. This toner image is transferred to an image transfer paper 35 fed from a paper feeding unit (not shown) by a transfer charger 36. Then, the image transfer paper 35 on which the toner image is transferred is subject to a fixing process done by a fixing unit (not shown) and then is ejected out of the apparatus.
On the other hand, toner left on the surface of the photosensitive body 31 after doing the transfer process is stripped off the surface of the photosensitive body 31 by a cleaning unit 37. The residual electric potential on the surface of the photosensitive body 31 is removed by a discharging lamp 38. Then, the operation goes to the next cycle.
The removal of the residual electric potential on the photosensitive body 31 after terminating the image transferring process by the discharging lamp 38 is an essential step. Except this discharging step, the electric potential on the photosensitive body 31 is going up by repetitively charging the electric potential on the surface by a Scorotoron charger 32. This results in bringing about insulation breakdown of the photosensitive body by the excessive charges charged on the photosensitive body 31.
Further, even when a discharging lamp or a ray of light for writing is used, the foregoing discharging needs to constantly light on the lamp for abruptly lowering the electric potential on the surface of the photosensitive body 31.
On the other hand, provision of a leased discharging unit like the discharging lamp 38 is a cause of inhibiting reduction and cost down of the unit. Hence, the arrangement providing no leased discharging unit, for example, an arrangement for doing discharge by a transfer charger has been proposed in the publication of Japanese Lying Open No. Showa 56-16155 or an arrangement for dividing a laser beam applied from the exposing unit and discharging the photosensitive body 31 by this laser beam.
However, the foregoing conventional arrangements do not overcome a shortcoming specific to a reversal developing system using two-component developing powder, that is, carrier or toner attraction on the photosensitive body 31 after forming an image. This shortcoming is an obstacle to forming an excellent image.
To overcome this shortcoming, as disclosed in the publication of Japanese Patent Lying Open No. Hei 3-23471, an electronic photography apparatus has been proposed to provide control means for controlling a power supply in a manner to switch off a developing bias voltage of the developing unit after applying such a voltage as enhancing the potential on a photosensitive body to a predetermined high level onto a main charger, applying such a voltage as dropping the potential on a photosensitive body to a low level onto a contact-transfer charger, and applying such a voltage as charging the surface of the photosensitive body with the electric potential level at which carrier attraction is inhibited, and exposing unit control means for operating an exposing unit in a manner to lower the electric potential on the surface of the photosensitive body up to a ground potential by means of a ray of light applied from the exposing unit.
In the conventional arrangement as disclosed in the Japanese Lying Open No. Hei 3-23471, no problem takes place if the contact-transfer charger is a non-contact type such as a Scorotoron type but the following problem takes place if it is a contact type like a transfer roller type.
After switching off the developing bias voltage, a slight amount of toner is attracted on the photosensitive body of the developing unit through an edge effect in the border (area immediately after the laser beam is switched on) when the surface potential of the photosensitive body is lowered to a value near the ground potential by means of a beam applied from the exposing unit.
In the case of a non-contact transfer type, a slight amount of toner on the photosensitive body is cleaned by a cleaning unit. In the case of a contact transfer type like a transfer roller type, the toner on the photosensitive body is attracted on the transfer roller so that dirt may be unfavorably printed on the rear surface of the image transfer paper.
If the surface potential is made lower, it is difficult to cut off the developing bias voltage in the state that the surface potential is completely matched to the bias as shown in FIG. 14. In actual, normally, both of the potential and the voltage may be slightly slipped as shown in FIGS. 15-2 and 15-3. In the case shown in FIG. 15-2, the disadvantage such as carrier attraction takes place. In the case shown in FIG. 15-3, the disadvantage such as toner attraction takes place. The occurrence of such disadvantages are known if the two-component system developing agent is used. This disadvantage brings about reduction of the developing agent or increase of a consumed amount of the developing agent.
In the case of using a contact member such as a transfer roller as a transfer member, the carrier attraction leads to impairing the photosensitive body, because the photosensitive body is laid between the transfer member and the photosensitive body. The toner attraction causes the toner to be attracted on the transfer member, which leads to a cause of dirt on the rear of the paper at the next print.
The present invention makes it possible to solve the foregoing disadvantages and supply an excellent print or copy with no dirt on the rear of an image transfer paper even in a contact-transfer system.